Spark optical emission spectrometry is a well known technique used to analyse solid samples. Optical emission spectrometry may be conducted with either a spark or arc for example. For convenience, as used herein, the term spark optical emission spectrometry means any optical emission spectrometry employing an electrical discharge to excite the sample such as a spark or arc for example, and the term spark chamber means a chamber for conducting any electrical discharge. A solid sample is typically mounted onto the table of a spark stand, also known as Petrey's stand. The spark stand further comprises a spark chamber, within which is an electrode oriented to present a tapered end towards the sample surface. The table of the spark stand has an aperture in the spark chamber wall over which the sample is mounted, usually with an air-tight seal. The electrode is surrounded by an insulator except for its tapered end. A sequence of electrical discharges is initiated between the electrode and the sample, in which the sample acts as a counter electrode. The insulator promotes discharge to the sample rather than the chamber wall. Sample material local to the discharges is vaporised and a proportion of the vaporised atomic material is raised to excited states. On relaxing, the atomic material emits photons, the energies of which are characteristic of the elements in the material. Spectroscopic analysis of the emitted photons enables the composition of the sample material to be deduced. The spectroscopic analysis is conducted using an optical analyser which usually utilises a dispersive means such as a grating to disperse light spatially according to its wavelength. A detector, such as an array detector for example, is used to measure the quantity of light as a function of the degree of dispersion. Hence, a proportion of the light emitted during the discharges is transmitted from the spark chamber to the analyser for spectroscopic analysis.
To obtain information about a wide range of elements within samples, the instrument must be capable of transmitting photons below 190 nm from the spark stand to the detector, as some elements emit photons in the ultraviolet (UV) wavelength range when relaxing to a lower energy state. To avoid absorption of these UV photons by air and to avoid wavelength shifts associated with changes in the refractive index of gases (which changes with the pressure of the gas and the gas composition), the sample material is excited in the presence of an inert gas, typically argon, which is fed into the spark chamber at least during the time when the sequence of spark discharges is initiated. The presence of an inert gas also prevents oxidation of the sample surface.
The electrical discharges cause material to be ablated from the sample surface and some of this material is not in atomised form. Some much larger aggregates or particles of material are removed from the sample surface which are useless for the spectroscopic process, and are referred to as debris. This debris, along with the vaporised atomic material is liberated from the sample surface at each electrical discharge. To prevent cross contamination or so-called memory effects, preferably all the ablated material from one sample should be removed from the spark chamber before analysis of the next sample to eliminate any re-deposition of material from a preceding sample onto the next sample, and to prevent any such material from being present in the path of the electrical discharges. The argon gas which bathes the sample and the discharge path is utilised to sweep ablated materials including debris from the spark chamber in a continuous or semi-continuous process. Argon gas is typically arranged to flow into the spark chamber through at least one gas inlet and out of the spark chamber through at least one separate gas outlet, the flow of gas sweeping debris and vaporised material from the chamber. The gas flow is arranged to be present during the time when the sequence of electrical discharge is initiated. The gas flow may also be present during the time between sequences of electrical discharges. It is important to avoid debris and vaporised sample material from depositing on the surfaces of adjacent optics, which would impair the transfer of photons from the sample region to the optically dispersing element of the spectrometer. Should this occur, the spectrometer would have to be shut down whilst the optics were cleaned. In some spark chambers the gas is introduced along a tube leading from the spark chamber to the spectrometer optics, with the gas flow directed away from the optics in order to reduce the likelihood that material will pass from the spark chamber and be deposited upon the surfaces of the optics.
For analysis of the nitrogen content of a sample, outgassing of residual nitrogen from material at the internal surfaces of the walls of the spark chamber has been found to cause instability of the recorded nitrogen signal, and inaccuracy of the measured result with a high background nitrogen signal recorded after insertion of a new sample. Performing sequences of electrical discharges promotes the outgassing of nitrogen by heating and irradiating the material on the chamber walls with UV radiation. Several sequences of electrical discharges must be made on a sample before this residual nitrogen reduces sufficiently and this is undesirable for high throughput instruments in which precise and reliable nitrogen analysis is desired from the first run. The presence of a flow of argon gas reduces the time for the residual nitrogen to reduce.
Hence the argon gas is utilised for several purposes. However, argon and other inert gases are expensive and contribute to the running costs of the spectrometer and it is desirable that the lowest possible flow of inert gas is used which will be adequate for the purposes described above.
U.S. Pat. No. 3,815,995 describes a form of gas injection which is coaxial with the pin-like electrode used in the discharge process. This means of gas injection was designed to reduce the spread in positions over the sample surface over which repeated electrical discharges take place. However this prior art method suffers from poor evacuation of debris from the spark chamber.
CN 1796983A and CN 2769882Y describe a spark chamber comprising two gas inlets, each arranged to provide a gas flow adjacent the internal wall of the spark chamber. This promotes a circular flow of gas within the chamber. This arrangement suffers from the disadvantage that the cyclone-type gas flow generated carries particulate material towards the chamber walls, where it accumulates, rather than sweeping it from the chamber.
JP10160674A2 describes four gas inlets which direct gas in an inward radial direction towards the pin electrode. The symmetrical disposition of the gas inlets promotes a more stable electrical discharge, but again evacuation of debris is not efficiently accomplished.
EP00398462B1 describes the use of pulses of purge gas through the spark chamber to more efficiently remove debris in between the electrical discharges. However this method may promote after-pulsing of residual gas flow which could carry particulate debris towards the collection optics, and thereby contaminate them.
In view of the above, the present invention has been made.